Frequency response characteristics of continuous-time filters depend on absolute values of capacitors and resistors or transconductances in the filters. The absolute values of these components in the monolithic integrated form tend to be highly variable. Because of this variability, tuning is often necessary to control the frequency response characteristics of continuous-time filters.
Various techniques are currently known for tuning the frequency response of continuous-time or analog filters. One of these methods is disclosed in "Fully Integrated Analog Filters Using Bipolar-JFET Technology" IEEE Journal of Solid-State Circuits, pp. 814-821, Vol., SC-13, No. 6, December 1978. It employs a real-time master-slave approach and involves the use of a phase-locked loop as shown in FIG. 7 of that reference on p. 817. However, that method is not effective for applications such as radio frequency receivers and portable wireless devices because it requires a significant amount of hardware and power dissipation. In addition, that method provides limited dynamic range for the filter because of the parasitic feed through of the reference signal during normal data transmission.
Another known solution to the problem of variability of filter component values is manual trimming. Manual trimming is unattractive because it requires additional manufacturing cost and it typically does not compensate for component aging.